It is well known that the connection, transmission and distribution technologies for the intermittent power sources such as wind energy and solar energy have become one of the preferred developing subjects in the current energy source field. Accordingly, the application of a battery energy storage system has gained wide attentions. An important component of the battery energy storage system is a power conversion system (PCS). The PCS has been widely applied in the distributed power generation technology such as solar energy and wind energy, and it has been gradually applied in the small-capacity two-way power transferring energy storage system. By means of the PCS, a two-way energy transfer between a DC battery of a battery energy storage system and an AC electric grid may be realized, so that the voltage control in the normal or island operation mode may be realized. An important role of the PCS is to, in the case that the electric grid is in outage, continue to provide a reliable voltage support to a load to meet the operation requirement of the load. Therefore, the study on the voltage and frequency control of the PCS becomes imperative under the situation.
The main object of controlling PCS voltage and frequency is to provide an expected voltage in an off-grid state. For a traditional PCS including a single-inductor filter, although its structure is simple, the filtering effect is poor. For a PCS including an LCL filter, the filtering effect is better than the above traditional PCS, but in the case of large capacity, it requires a high DC-side voltage, which is adverse to the series/parallel connection of the DC-side battery components.
As a power conversion system, the PCS may realize energy storage and the two-way energy flow between the electric grid and the load. In order to improve the output performance of the PCS, the existing PCS often includes an LCL-type filter or an isolating transformer. Thus, the mathematical model of the PCS will become a high-order model. When the traditional voltage-current double closed-loop control is employed, not only more voltage-current sensors are required, but also the number of PI controllers (proportional integral controllers) is increased. Thus, it is difficult to debug and inconvenient for engineering application.